1. Field of the Invention
The present invention relates to a television (hereinafter, abbreviated as "TV") projector for projection display and, more specifically, to an improved TV projector for so-called three-tube projection display employing three projection TV cathode-ray tubes (hereinafter, abbreviated to "CRTs") for projecting red, green and blue color pictures, respectively.
2. Description of the Prior Art
Shown in FIG. 4 is a conventional three-tube rear projection display system employing a red picture projection TV CRT (hereinafter, referred to as "R projection TV CRT") 10R for projecting a red picture, a green picture projection TV CRT (hereinafter, referred to as "G projection TV CRT") 10G for projecting a green picture, and a blue picture projection TV CRT (hereinafter, referred to as "B projection TV CRT") 10B for projecting a blue picture, and three projection lenses 20R, 20G and 20B for focusing the red, green and blue pictures, respectively, on the back surface of a screen 30 so that the red, green and blue pictures are synthesized in an enlarged color picture which can be seen on the front surface of the screen 30.
The R projection TV CRT 10R, the G projection TV CRT 10G and the B projection TV CRT 10B have different luminescent characteristics, respectively. The luminescent characteristic of the G projection TV CRT 10G, in particular, has a sideband in the vicinity of 580 nm and 625nm as indicated by a continuous line A in FIG. 5, which are in the region of the red frequency band. Since this sideband reduces the color purity of the green picture, the green picture projected by the G projection TV CRT is tinged with yellow and is displayed in an yellow-green picture. When the red picture, the blue picture and the yellow-green picture are synthesized on the screen 30, a color picture of unsatisfactory color balance is displayed on the screen 30. Accordingly, a component lens of the G projection lens 20G is coated with a dichroic filter having a filtration characteristic for showing light transmissivity thereof as indicated by an alternate long and short dash line B in FIG. 5 to improve the color purity of the green picture.
However, when the G projection lens 20G including a component lens coated with a dichroic filter is used, light cut off by the dichroic filter is reflected by the G projection lens 20G projection TV CRT 10G to illuminate the front glass plate of the G projection TV CRT to deteriorate the contrast of the green picture. Furthermore, since the filtration characteristic of the dichroic filter is dependent on the angle of incidence of light, the filtration characteristic varies with the angle of incidence of light causing color shade in the color picture displayed on the screen.